1. Field of the Invention
The present invention generally relates to an adsorptive honeycomb-shaped ceramic structure and a method for its production, and particularly relates to a honeycomb-shaped ceramic structure having a rich adsorption property and favorably used as a drying agent which generates dry air for drying granules, as in a microwave drying apparatus for drying granules, and a method for its production.
2. Description of the Related Arts
Conventionally, there has been provided an apparatus to utilize hot wind in order to dry the powder granules containing water or a low boiling point solvent. Because it is inefficient to cool down the circulating air at a high temperature with such an apparatus, it is desirable that the air at a relatively high temperature of about 50.degree.-100.degree. C. be used and circulated in case of cooling. Accordingly, in order to generate such a dry air, a drying agent having a large moisture adsorbing capacity even in the above temperature range is desired.
As a method for generating the dry air, there is one employing a plurality of rotors respectively filled with granular drying agent according to the purpose of the air used, while, as another method for generating the dry air, there is one employing a honeycomb columnar rotor as drying agent having a honeycomb-like structure, and the latter method is considered more convenient. According to this latter method, while the rotor is rotated at a slow speed of 3-10 times an hour, air is passed therein to have the rotor adsorb moisture to obtain dehumidified air. According to the honeycomb rotor system, the columnar honeycomb rotor is divided into three angular regions: the first region through which the air to be dried is passed; the second region through which heated air (hot wind) is passed to heat and dehumidify the drying agent, i.e. rotor itself, which have absorbed moisture in the first region; and the third region through which cold air is passed to cool the drying agent, i.e. rotor itself, which revived in the second region, thereby carrying out a cycle of dehumidifying the air, reviving the drying agent, i.e. rotor itself, and the cooling the agent, repeatedly. This method has an advantage of allowing the drying agent to revive easily and of being able to get stabilized dry air without unstableness in dehumidifying.
As to the drying agent to be used for these dryers, silica gel or zeolite is well known. Silica gel has a satisfactory moisture adsorption capacity at a low temperature, while it has an insufficient moisture adsorption capacity at a relatively high temperature. Further, in case of lowering the temperature of the dry air generated in the dryer so as to improve the moisture adsorption property of silica gel by using water, it is almost impossible to cool it down below 35.degree. C. in summer season, and the silica gel does not exhibit its adsorption capacity sufficiently at the temperature. Accordingly, silica gel does not meet the condition required as a drying agent in use for a drying apparatus employing the microwave heating.
On the other hand, as zeolite (molecular sieve) has an excellent moisture adsorption capacity at a relatively high temperature, it is useful as a drying agent for a dryer utilizing microwave heating. However, it has problems in processability; accordingly, normally it is used in a granular form. In this case, as the adjacent portions between the zeolite particles do not necessarily constitute the effective adsorption surface area as a drying agent, the effective surface area is not sufficient. Accordingly, it is desired to make the effective surface area larger in order to improve dehumidifying capacity.
As a drying agent improved so as to make the effective adsorption surface area of zeolite larger, there is known a drying agent comprising a structure of a ceramic sheet, formed in honeycomb shape, with coating of zeolite powder, which is used as a honeycomb rotor. However, this drying agent has several defects as follows. Firstly, it is difficult to produce it due to its distinctiveness that the ceramic sheet is processed into a corrugated board by a special treatment such as a corrugate processing, and then the zeolite powder is coated on the surface of the honeycomb structure formed into a roll shape by winding a ceramic sheet. Second, its cost is high. Third, if it is the type of dryer employing the honeycomb rotor, reviving the drying agent, i.e. the honeycomb rotor itself, by hot air at about 200.degree. to 250.degree. C. and cooling down the revived drying agent to a room temperature by air are carried out repeatedly. However, if this cycle continues, there arise such problems as the zeolite falling off the surface of the ceramic sheet, or degradation thereof by thermal impact or the like, leading to lowering of durability as a drying agent.
In addition to the abovementioned drying agent, as honeycomb-shaped ceramic structures, there are known a honeycomb structure of an extrusion molding product, a three-dimensional net-like structure (a spongy net), or a ceramic porous structure, having a multi-layer structure, made by regularly multi-laying linear extrusion product one above the other spirally (Japanese Laid-Open Patent (Unexamined) Publication No.242980/1988). This structure may be utilized for a filter of molten metal or catalyst carrier, but is not used as a drying agent. Heretofore, there has not been known any such drying agent made by extruding the composition containing zeolite powder into a honeycomb shape.